How to Determine Molar Solubility

Molar solubility problems come up in the study of solubility equilibria and the solubility product constant in the first year of General Chemistry. Molar solubility just means the solubility in units of moles per liter (mol/L). To determine this solubility we use the solubility product constant (Ksp), many of which can be found in Tables. By using these tools we can determine degrees of solubility.

Instructions

Difficulty: Moderately Challenging

1. Set up your equilibrium equation with your reactant on the left and your products on the right. Note whether the ionic compounds are solid, aqueous, liquid or gas.
2. Use the given solubility product constant (Ksp) from a Table that can be found in a good chemistry book. Note there are ways to determine a Ksp from scratch, but today we are simply determining molar solubility.
3. Note that solids and liquids can be deleted from the equation. Because the solubility of a solid or liquid is not going to change.
4. Set up your ICE table. This is table where you note the compound at the top of the table. On the left hand side you can write the words Initial, then Change under that, and then Equil(ibrium) under that. Taking the first letter of each, it spells ICE.
5. Take AgCl goes to Ag + Cl for instance. The compound is neutral, the Ag is 1 positive, the Cl is 1 negative. The compound is a solid, so ignore it. The products are aqueous.
6. Note '0' for Initial for both Ag and Cl. Note +S for both for Change. 'S' represents our molar solubility. Then add up the '0' and the +S's and you get 'S' for each, Ag and Cl.
7. Do your calculations now. Ksp equals the two products times each other, [Ag] x [Cl], which equals the S x S, as noted in Step 6. This equals S^2. In this case, Ksp from the table is 1.77 x 10^-10. Square root of S^2 is S. Square root of Ksp is 1.33 x 10^-5 M, mol per liter. And that is the molar solubility of AgCl.